1. Field of the Invention
The present invention relates to a photoelectric conversion device and an imaging device including it.
2. Description of the Related Art
FIG. 7 is a circuit diagram schematically showing the arrangement of a photoelectric conversion device. A photoelectric conversion device 200 shown in FIG. 7 includes a pixel 10 which outputs a signal to a vertical signal line 107, and an amplifier 11 which amplifies a signal supplied via the vertical signal line 107.
The pixel 10 includes a photodiode 101, transfer switch 102, reset switch 103, amplification transistor 104, and row selecting transistor 105. The transfer switch 102 transfers a charge generated by the photodiode 101 to a floating diffusion (FD) 106 when a transfer pulse φTX is enabled. The amplification transistor 104 constitutes a source-follower circuit with a constant current load 111 connected to the vertical signal line 107, and outputs a signal to the vertical signal line 107 in accordance with the potential of the FD 106. The reset transistor 103 resets the FD 106 and the photodiode 101 when a reset pulse φRES is enabled. The row selecting transistor 105 connects the source of the amplification transistor 104 to the vertical signal line 107 when a row selecting signal φSEL is enabled. That is, when the row selecting transistor 105 connected to the amplification transistor 104 is activated, that is, when the row to which the pixel 10 belongs is selected, the amplification transistor 104 outputs a signal to the vertical signal line 107.
FIG. 5 is a view schematically showing the structure of a MOS transistor. A parasitic capacitance (overlap capacitance) is formed between a gate and a diffusion region (source or drain) via a gate oxide film. This parasitic capacitance causes capacitive coupling between the gate and the diffusion region (source or drain) to transmit the potential fluctuation of the gate to the diffusion region at a predetermined ratio. The transmission ratio depends on the size of the parasitic capacitance. The parasitic capacitance can be controlled in accordance with a device structure. For example, the parasitic capacitance is several tens of aF to several fF per unit gate width.
The transfer switch 102 comprises a MOS transistor, and includes a parasitic capacitance 108. When the transfer pulse φTX is enabled to high level, the parasitic capacitance 108 fluctuates the potential of the FD 106. Hence, the potential of the vertical signal line 107 fluctuates by a value obtained by multiplying the potential fluctuation of the FD 106 by the gain of the source-follower circuit including the amplification transistor 104 and the constant current load 111. As a result, the output from the amplifier 11 falls outside a normal operation range. This phenomenon is called a range over. After that, it takes a long time before the output from the amplifier 11 returns to the normal operation range, thus prolonging a readout time. As a result, for example, the number of frames to be photographed per second decreases in a digital still camera, the frame rate decreases in a video camera, and an image capturing time becomes long in an image scanner image input device. Even if the gain of the amplifier 11 is low and the range over does not occur, a pseudo signal fluctuates the output from the amplifier 11, thus prolonging the readout time.